The present invention relates generally to an electrohydraulic control system for a hydraulic system, and more particularly relates to an interface between a hydraulic system and an electronic controller.
When controlling a hydraulic system with electrical signals, it is common practice to make the system control valves pressure responsive and control their movement through the use of solenoid operated pilot valves. However, all of these systems have had one or more limitations which are unacceptable in some applications. For example, many systems required two solenoid pilot valves for each control valve employed in the system and others required a constant electrical current applied to the solenoid pilot valves in order to maintain control valve position. Those systems requiring two solenoid pilot valves for each control valve could be prohibitive in cost and were often unreliable due to the number of solenoids, which are the most unreliable elements in the system. Those systems requiring constant electrical current to maintain control valve spool position are unreliable, since the constant current creates excessive heat, which tends to deteriorate the solenoid. Systems requiring a constant electrical current to maintain control valve position also present a safety hazard in that in the event of an electrical failure condition such as a power failure, or short or open circuit condition, the condition of the control valves would change unexpectedly. An example as to how this could be a definite safety hazard would be in the control of a planetary transmission with hydraulically controlled brakes and clutches to establish the different speed ratios. If the transmission and engine were being used to provide a braking force while traveling down a hill, a loss of electrical power could cause the transmission to shift into neutral, resulting in a loss of the braking force.
In some electro-hydraulic control systems, it was possible to operate one or more solenoid pilot valves when another was inoperative, due to a short circuit in the control wires or a defective solenoid. This also created a safety hazard in that it was possible to wind up with a hydraulic system condition which is other than that asked for by the operator.